Networking From the Rooftop

A few weeks ago, MIT graduate student Shan Sinha canceled his broadband Internet service. Now his Net connection comes through the chimney. From a computer in the living room of his Cambridge, MA, apartment, a few blocks from the MIT campus, a cable goes into the fireplace up to the roof, where it is attached to an antenna. From there, data packets hop to another roof-mounted antenna at a nearby student’s apartment. That way, from roof to roof in multiple hops, Sinha’s data packets finally reach a gateway-a computer connected to the fixed Internet-at MIT’s computer science building. “We can’t use the fireplace,” he says, “but that’s the cost of free Internet.”

Sinha’s “chimney connection” is part of MIT’s Roofnet, a project to create a self-organizing wireless network in which an amorphous, unmanaged collection of cheap Linux computers equipped with Wi-Fi cards collaborate to efficiently route data packets. Each computer and roof-mounted antenna at students’ apartments and MIT buildings is a node on the network and the arrangement in which they are connected to each other-the topology of the network-is constantly changing. “We want to understand how a whole bunch of computers with short-range radios can self-configure a network, forming order out of chaos,” says computer science professor Robert Morris, who coordinates the project. The network has now more than 30 nodes in a 4-square kilometer area surrounding the MIT campus. “We hope to reach a hundred nodes within a few months,” he says.

Research groups at universities such as Carnegie Mellon, Rice, UCLA, and the University of Illinois at Urbana-Champaign, and at companies such as Nokia, Intel, and Microsoft are developing similar systems. In each case, data packets are routed through geographically dispersed and wirelessly connected nodes that can be fixed in a building or moving with a user or vehicle. Applications of these so-called multi-hop mesh networks include systems to connect people carrying PDAs, tanks on a battlefield, or a large number of sensors in a factory plant. And community mesh networks such as Roofnet, which are much cheaper to deploy than DSL or cable hookups, are a promising way to overcome the “last mile” barrier and bring high-speed Internet access to a large number of people, especially those who live in rural areas or other places where the infrastructure for wired broadband access is not available.

Community-owned wireless networks have appeared in several places in New York, San Francisco, Seattle, London, and other cities. These networks usually consist of a few interconnected base stations-known as wireless access points-located in windows and rooftops providing Internet connectivity in public spaces. The new generation of mesh networks such as Roofnet cover wider areas and are much more dynamic in the way they route data. Their nodes are not permanently connected; instead, they constantly revaluate the existing links and form new ones. As a result, data follows much more tortuous paths to reach the fixed Internet. And with tens or hundreds of nodes-some of them joining and leaving the network in a random fashion and thus constantly changing its topology-a difficult problem arises: how should data in these multi-hop wireless nets be routed? What paths in this labyrinth of rooftop and window antennas optimize the flow of packets?